1. Technical Field
This application generally relates to data storage, and more particularly to caching techniques.
2. Description of Related Art
Computer systems may include different resources used by one or more host processors. Resources and host processors in a computer system may be interconnected by one or more communication connections. These resources may include, for example, data storage devices such as those included in the data storage systems manufactured by EMC Corporation. These data storage systems may be coupled to one or more host processors and provide storage services to each host processor. Multiple data storage systems from one or more different vendors may be connected and may provide common data storage for one or more host processors in a computer system.
A host processor may perform a variety of data processing tasks and operations using the data storage system. For example, a host processor may perform basic system I/O operations in connection with data requests, such as data read and write operations.
Host processor systems may store and retrieve data using a storage device containing a plurality of host interface units, disk drives, and disk interface units. Such storage devices and data storage systems are provided, for example, by EMC Corporation of Hopkinton, Mass. The host systems access the storage device through a plurality of channels provided therewith. Host systems provide data and access control information through the channels to the storage device and storage device provides data to the host systems also through the channels. The host systems do not address the disk drives of the storage device directly, but rather, access what appears to the host systems as a plurality of logical disk units, logical devices or logical volumes. The logical disk units may or may not correspond to the actual physical disk drives. Allowing multiple host systems to access the single storage device unit allows the host systems to share data stored therein.
Caching may be used in data storage systems to improve performance. Different techniques may be used to store and/or retain in cache the data which is expected to be referenced at a future point in time. When servicing a data request, a system may first determine whether the requested data is in cache. If so, then the data request may be serviced using the cached data thereby avoiding the additional time required to retrieve the requested data from a physical device. However, if the requested data is not in cache, the requested data is retrieved from the physical device, may be stored in the cache, and is used to service the data request. The amount of time to service the data request may be reduced when the requested data is already in cache rather than having to wait the additional time to retrieve the requested data from a physical device.